The present invention relates to an inspection method and apparatus using an electron beam and, more particularly, to an inspection method and apparatus suitable for inspection of a pattern formed on the surface of a semiconductor wafer.
Conventional scanning electron microscopes such as the one shown in FIG. 1 observe and inspect samples such as a semiconductor wafer or the like by irradiating an electron beam onto the sample surface. An electron beam is emitted by a cathode 35, is focused by a focusing lens 36 and objective lens 38, and is scanned over the surface of the grounded sample 39 by a scanning electrode 37.
A point light source is normally used for the cathode 35. In order to obtain a resolution that allows pattern observation, the electron beam emitted by the point light source is temporarily focused by, e.g., a focusing lens 36, and is focused again by an objective lens 38 to reduce the size of the spot. The scanning electrode 37 scans an electron beam over the surface of the sample 39. Secondary and reflected electrons are emitted from the surface of the sample 39, and are detected and converted into a detection signal by a detector 40. The detection signal is amplified by an amplifier 41, and is then supplied to an image processor 43. A sync. signal for synchronizing the detection signal with the scans of the electron beam is supplied from a scanning circuit 42 to the image processor 43. The luminance of the monitor 44 is determined by the amount of information indicated by the detection signal. This signal is synchronized by the sync. signal and output from the amplifier 41. In order to increase the S/N ratio of the image, the image signals for each frame output from the amplifier 41 are combined by the image processor 43, and the resultant image is displayed on the monitor 44.
An example of a conventional electron beam inspection apparatus is shown in FIG. 2. This apparatus was proposed by Japanese Patent Laid-Open No. 5-109381 and is balled a direct reflected electron microscope. A primary electron beam emitted by an electron gun 1 is focused by an irradiation lens system 6, and is then deflected by a first Wien filter 3 (that applies an electric field and a magnetic field to the electron beam), before hitting the surface of the sample 4 at perpendicular angle. Secondary and reflected electrons emitted from the sample 4 are accelerated by an emission lens 5. The electron beam is then projected onto the first screen 7 by the first projection lens system 2, thus allowing the operator to directly observe the projected image.
Energy analysis can also be made. To do this, the first screen 7 is removed from the optical axis and the second Wien filter 8 is set to only allow secondary and reflected electrons of a certain energy level to pass straight through. The secondary and reflected electrons that pass straight through the second Wien filter 8 are enlarged to a predetermined size by a second imaging lens system 9, and are displayed on the second screen 10.
Such conventional electron beam inspection apparatuses, however, suffer from the following problems. The scanning electron microscope shown in FIG. 1 uses a scanning electrode, scanning coil to allow the operator to observe a wide range of the sample surfaces while attaining suitable resolution. However, the speed of scanning is limited in this method since the linearity of scanning must be maintained. Furthermore, since the electron beam must be focused, the current amount decreases, resulting in a drop in the S/N ratio.
Conventionally, in order to solve such problem, images formed by secondary and reflected electrons are processed and stored in a memory, and are combined in single frame unit. However, in this method, the image display speed decreases.
Furthermore, upon deflecting the electron beam to scan a broader surface area, the electron beam shifts from the center of the lens optical axis, thereby producing lens aberrations, and resulting in poor resolution.
In the direct reflected electron microscope shown in FIG. 2, a primary electron beam is generated by a standard electron gun 1 and irradiation lens system 2. In this arrangement, however, the electron beam cannot cover a sample region larger than 200 .mu.m.sup.2 at once. When the magnification must be changed in order to inspect a new region, the size of the primary electron beam must also be changed. However, in a conventional electron microscope, the beam shape cannot be changed without decreasing the current density. Furthermore, secondary and reflected electrons passing through the emission lens 5 are affected by the aberrations and transmittance of the emission lens 5. Such influences, however, are not taken into consideration.